1. Field of the Invention
The present invention relates to a method and an apparatus for detecting a dynamic change in ultrasonic wave or the like propagating through a medium. Furthermore, the invention relates to an ultrasonic diagnostic apparatus having such a dynamic change detecting apparatus.
2. Description of a Related Art
In an ultrasonic diagnostic apparatus for so-called ultrasonic echo observation or the like, it is the general practice to use a piezo-electric material typically represented by PZT (Pb (lead) titanate zirconate) for an ultrasonic sensor section (probe).
FIGS. 11A and 11B schematically illustrate the structure of a conventional probe: FIG. 11A is a whole perspective view of the probe, and FIG. 11B is an enlarged perspective view illustrating an array oscillator.
The probe 301 has a thin box shape as a whole and has a long and slender rectangular probing surface 302. This probing surface 302 is brought into contact with a human body and an ultrasonic wave is radiated so as to receive an ultrasonic echo reflected from the depth of the body. In FIG. 11A, a cable 307 sending an ultrasonic receiving signal is connected to the upper side of the probe 301.
A comb-shaped array oscillator 303 serving simultaneously as a transmitter and a receiver of ultrasonic wave is housed in the probing surface 302. The array oscillator 303 is formed by providing a number of slits 306 (having a width of, for example, 0.1 mm) in a thin (having a thickness of, for example, 0.2 to 0.3 mm) strip-shaped PZT sheet and arranging many (for example, 256) comb-teeth-shaped individual oscillators 305 (having, for example, a width of 0.2 mm and a length of 20 mm).
Electrodes are formed in each individual oscillator 305, and signal lines are connected thereto. An acoustic lens layer or an acoustic matching layer made of a resin material such as rubber is pasted to the surface side (lower side in FIG. 11A) of the array oscillator 303, and a packing material is pasted onto the back side. The acoustic lens layer converges the transmitted ultrasonic waves effectively. The acoustic matching layer improves the transmission efficiency of ultrasonic waves. The packing material has a function of holding the oscillator and causes oscillation of the oscillator to be finished early.
These ultrasonic probes and ultrasonic diagnostic apparatuses are described in detail in the xe2x80x9cUltrasonic Observing Method and Diagnostic Methodxe2x80x9d, Toyo Publishing Co., and xe2x80x9cFundamental Ultrasonic Medicinexe2x80x9d, Ishiyaku Publishing Co.
In the area of ultrasonic diagnosis, collection of three-dimensional data is demanded for obtaining more detailed information about the interior of an object""s body. In order to comply with such a demand, it is required to make ultrasonic detecting elements (ultrasonic sensors) into a two-dimensional array. In the aforementioned PZT, however, refinement and integration of devices over the present status is difficult for the following reasons. That is, the processing technology of PZT materials (ceramics) is on a limit level, and further refinement leads to an extreme decrease in processing yield. This furthermore results in an increase in the number of wires, thus leading to an increase in electrical impedance of wiring. In addition, there is an increase in crosstalk between individual elements (individual oscillators). It is therefore considered difficult at the present level of art to achieve a two-dimensional array probe using a PZT.
A paper entitled xe2x80x9cProgress in Two-Dimensional Arrays for Real-Time Volumetric Imagingxe2x80x9d by E. D. Light et al., Duke University, appears in ULTRASONIC IMAGING 20, 1-15 (1998), disclosing a probe having a two-dimensional array of PZT ultrasonic sensors. The paper however states xe2x80x9cIn order to obtain an image of a similar quality, it is necessary to provide 128xc3x97128=16,384 elements of the two-dimensional array. However, it is complicated and expensive to make such many RF channels, and therefore, there is only a little chance of this solution in the future. It is furthermore very difficult to densely connect such many elements.xe2x80x9d (page 2, lines 14-18).
On the other hand, a sensor using optical fibers is used as an ultrasonic sensor not using a piezo-electric material such as PZT. Such an optical-fiber ultrasonic sensor is suitable for measurement at a location largely affected by magnetic field or at a narrow site.
There is available a kind of optical fiber ultrasonic sensor which uses an optical fiber Bragg grating (hereinafter abbreviated as an xe2x80x9cFBGxe2x80x9d) (see TAKAHASHI, National Defense Academy, et al. xe2x80x9cUnderwater Acoustic Sensor with Fiber Bragg Gratingxe2x80x9d OPTICAL REVIEW, Vol. 4, No. 6 (1997) 691-694). An FBG is formed by alternately laminating two kinds of material layers (light propagating medium) having different values of refractive index in several thousand layers so that refractive index changes periodically at a pitch satisfying Bragg""s reflection conditions. The pitch of the periodic structure is xcex94, a wavelength of incident light is xcex, and N is an arbitrary integer, then, Bragg""s reflecting condition is expressed by the following formula:
2Nxcex94=xcex
Under the action of Bragg reflection, the FBG selectively reflects light having a particular wavelength satisfying the above formula, and causes light having the other wavelengths to pass through.
When an ultrasonic wave is caused to propagate to FBG, the wavelength xcex of the light selectively reflected changes because deformation of FBG leads to change in the pitch xcex94 of the aforementioned periodic structure. In practice, there are slant zones in which the reflectance varies before and after a central wavelength showing the highest reflectance (lowest transmittance), and an ultrasonic wave is applied to the FBG while a detection light having a wavelength in these slant zones is made incident to the FGB. It is thus possible to observe change in intensity of the reflected light (or transmitting light) corresponding to intensity of the ultrasonic wave. The ultrasonic intensity can be determined by converting this change in light intensity into an electric signal.
There is available another kind of optical fiber ultrasonic sensor using a Fabry-Perot resonator (hereinafter abbreviated as an xe2x80x9cFPRxe2x80x9d) (see UNO et al., Tokyo Institute of Technology, xe2x80x9cFabrication and Performance of a Fiber optic Micro-Probe for Megahertz Ultrasonic Field Measurementsxe2x80x9d, T. IEE Japan, Vol. 118-E, No. 11, ""98).
The sensor of UNO et al. is prepared by forming a half mirror through vapor deposition of gold at the leading end of a single-mode optical fiber (xcex=1.3 xcexcm, core: 10 xcexcm and clad: 125 xcexcm), providing a cavity (length: 100 xcexcm) by a member of polyester resin (n=1.55) at the leading end thereof, and forming a total reflection mirror by gold vapor deposition further at the leading end thereof.
Detection light having a wavelength xcex is made incident into this sensor from the half mirror side, and an ultrasonic wave is transmitted from the total reflection side. When the reflectance of the half mirror is r, the single pass gain is G, the length of the cavity is L, and the refractive index is n, then, the reflectance R of this sensor is determinable from the following formula:   R  =                              (                                    r                        -            G                    )                2            +              4        ⁢                  r                ⁢        G        ⁢                  xe2x80x83                ⁢                  sin          2                ⁢        δ                                      (                      1            -                                          r                            ⁢              G                                )                2            +              4        ⁢                  r                ⁢        G        ⁢                  xe2x80x83                ⁢                  sin          2                ⁢        δ            
where, xcex4 is calculated by means of the following formula:
xcex4=2xcfx80Ln/xcex
The formula expressing xcex4 suggests that change in an optical path length 2L of a reciprocation of the cavity caused by change in sound pressure of the ultrasonic wave, i.e., change in the optical path length L leads to change in the reflection property of the light from the sensor.
In practice, there are slant zones in which the reflectance varies before and after the central wavelength giving the lowest reflectance, and change in intensity of the reflected light corresponding to the intensity of ultrasonic wave can be observed by applying an ultrasonic wave to the FPR while the detection light of any of the slant zones into the FPR is made incident. The intensity of ultrasonic wave can be determined by converting this change in intensity of the light.
The aforementioned ultrasonic sensor using optical fiber has however a defect. If steep characteristics of the slant zones are designed with a view to improving the detection sensitivity, the dynamic range becomes inevitably narrower. Design of slower characteristics in contrast results in a wider dynamic range, but in poorer detection sensitivity.
The present invention was developed in view of these problems. The invention has a first object to permit, in the detection of a dynamic change, selection of various combination conditions of detection sensitivity and dynamic range. A second object of the invention is to make it possible to detect a dynamic change in parallel under a plurality of combination conditions.
For the purpose of solving these problems, the dynamic change detecting method according to the present invention comprises the steps of: (a) entering light having at least one wavelength component from one end of a transmission/reflection part having transmission/reflection properties varying with a wavelength of incident light into a detecting element having a detecting part composed by connecting a resonant part including a medium having a predetermined thickness and a total reflection mirror to the other end of the transmission/reflection part; (b) detecting the reflected light by means of the detecting part to obtain a detection signal; and (c) detecting dynamic change propagating to the total reflection mirror, on the basis of change in amplitude of the detection signal corresponding to change in size of the detecting part caused by propagation of the dynamic change.
The dynamic change detecting apparatus according to the present invention comprises a light source for emitting a light having at least one wavelength component; a detecting element having a detecting part composed by connecting a resonant part including a medium having a predetermined thickness and a total reflection mirror to one end of a transmission/reflection part having transmission/reflection properties varying with a wavelength of incident light, light emitted by the light source being entered from the other end of the transmission/reflection part into the detecting element; a plurality of light detectors for detecting the light reflected by the detecting part to detect a detection signal; and a signal processing unit for detecting dynamic change propagating to the total reflection mirror, on the basis of change in amplitude of the detection signal corresponding to change in size of the detecting part caused by propagation of the dynamic change.
The ultrasonic diagnostic apparatus according to the present invention comprises a transmitting unit for transmitting an ultrasonic wave to an object; a light source for emitting a light having at least one wavelength component; a detecting element having a detecting part composed by connecting a resonant part including a medium having a predetermined thickness and a total reflection mirror to one end of a transmission/reflection part having different transmission/reflection properties in response to a wavelength of incident light, the light emitted by the light source being entered from the other end of the transmission/reflection part to the detecting element; a plurality of light detectors for detecting the light reflected by the detecting part to obtain a detection signal; a signal processing unit for detecting dynamic change propagating to the total reflection mirror, on the basis of change in amplitude of the detection signal corresponding to change in size of the detecting part caused by propagation of the dynamic change; and an image display unit for displaying an image on the basis of the signal output from the signal processing unit.
According to the invention, there is used a detection element having a detecting part formed by connecting a resonant part including a medium having a predetermined thickness and a total reflection mirror to one end of a transmission/reflection part 3 having transmission/reflection properties varying with a wavelength of incident light. It is therefore possible to achieve simultaneously high detection sensitivity and a wide dynamic range by changing the wavelength of the incident light.